The present invention relates to a method and apparatus for detecting pulse density violations in the T1 transmission of digital signals.
In the transmission of digital signals, the information is transmitted in frames and synchronized by a process called framing. Each frame contains 193 bits of data with 24 frames making up an extended super frame of 4,632 bits. Some of the bits are used for synchronization and some for error detection. The common North American standard for the transmission of T1 information is specified in ANSI T1.403-1989 which provides that there be a minimum of N "ones" in each and every time window of 8(N+1) time slots or bits where N can equal from 1 to 23. The time windows defined by the ANSI standard consist of a smallest window of two octets covering bits A.sub.0 to A.sub.15. The largest window contains 24 octets covering the most recent 192 digit time slots of A.sub.0 to A.sub.191.
One way of ensuring the integrity of the transmitted data and being able to recover a clock from the incoming data is to ensure that there are a sufficient number of "ones" in a 192 bit window or time frame and monitoring the data to see that it has the required number of "one" bits. Thus, the ANSI T1 standard ensures that one will be able to recover a clock from the incoming data as well as prescribing a standard for the industry. Presently, the only known methods of monitoring the number of "ones" and ensuring that there are at least N "ones" in 8(N+1) time slots is to record a violation and maintain that violation status until all 193 good bits comes through. Such schemes do not allow bit by bit monitoring and identification of both the offending bit (i.e. the bit that puts the window into violation) and the saviour bit (i.e. the bit that corrects the violation). Bit-by-bit monitoring would allow "stuffing" of a correction bit into the required location, as well as better performance monitoring.
The only known technique used to detect and report the presence and absence of pulse density violations is combinational logic. However, such a technique requires an enormous number of input combinations and makes the task difficult.
Accordingly, it is an object of the invention to provide an improved method of detecting pulse density violations. It is a further object of the invention to provide a simpler method of detecting pulse density violations.